1. Field of Invention
The present invention relates to a technical field involving the use of electron-transfer materials, and more particularly, to a novel electron-transfer compound as well as to a process for producing such a compound. The present invention further relates to an electron-transfer agent containing such a compound, and also, to an electrophotographic photoreceptor and an organic electroluminescence element containing such an agent.
2. Description of Related Art
Photocopiers, laser printers and other electrophotographic apparatuses are equipped with an electrophotographic photoreceptor. In the early days of development of photocopiers and laser printers, inorganic film was used in the photosensitive layer of the electrophotographic photoreceptors. Such inorganic film was formed of inorganic materials such as selenium, selenium-tellurium, selenium-arsenic and amorphous silicon.
As demand increases for inexpensive, environmentally less harmful electrophotographic photoreceptors, the photoreceptors incorporating organic film have become widely used and have replaced those with conventional photosensitive layers formed of inorganic film. The photosensitive layers formed of organic film are generally divided into two structurally different types: single-layered and multi-layered.
The single-layered photosensitive layer comprises a single layer of charge-transfer medium in which a charge-generation material has been dispersed. The single layer serves both to generate electrical charge and to transport charge. In comparison, the multi-layered photosensitive layer is formed as a multi-layered film comprising a charge-generation layer (CGL) and a charge-transfer layer (CTL) that are laminated on top of one another. The two layers have different functions with the charge-generation layer generating electrical charge and the charge-transfer layer transferring the generated charge.
While both types of the photosensitive layers are in use today, each requires a charge-transfer material with a high mobility in order to increase sensitivity.
The organic photosensitive layers are also divided into two different types, namely, positive charge photosensitive layers and negative charge photosensitive layers, based on the polarity they can be charged as well. Most of the known charge-transfer materials that have high mobility and are in practical use today are hole-drift type charge-transfer materials. Accordingly, the photoreceptors used in commercial electrophotographic products employ a negative charge photosensitive layer.
When these photosensitive layers are negatively charged by means of corona discharge phenomena, significant amounts of ozone are produced, causing many problems such as pollution of indoor environments and accelerated deterioration of the electrophotographic photoreceptor.
In order to avoid these problems occurring during the negative charging process, conventional electrophotographic apparatuses employ ozone filters or a special ozone-free charging technique. These approaches, however, bring about new problems, such as resulting in an undesirably large construction of the apparatus or complex electrophotography process. Further, none of these approaches has ever provided a practical solution.
As a result, the positive charge photoreceptors, which produce little ozone, are demanded in the marketplace as an effective countermeasure to the above-described problems, and to this end, a highly mobile electron-transfer material must be developed that can be used in the positive charge photosensitive layer.
Negative charge photoreceptors are better suited for use in color printers because of available toners. Also, by constructing the photosensitive layer as a single layer in a negative charge photoreceptor, the time required for the coating process, and thus the production cost, can be reduced. Constructing such a photosensitive layer, however, requires an electron-transfer material with even higher mobility, and no material has ever been found to have a sufficiently high electron mobility to provide such characteristics.
Therefore, a highly mobile electron-transfer material is as important in the negative charge photoreceptor as it is in the positive charge photoreceptor. Much effort has been made to find such material. The electron-transfer materials for use in the positive charge photoreceptor that are known to date include trinitrofluorenon (TNF), tetracyanoethylene, tetracyanoquinodimethane (TCNQ), quinone, diphenoquinone, naphthoquinone, anthraquinone, and derivatives thereof. Most of these electron-transfer materials, however, have a poor compatibility with binder resins so that it is difficult to uniformly disperse these materials in a photosensitive layer at a high concentration. Thus, the amount of the electron-transfer material contained in the photosensitive layer tends to be too small to provide sufficient electrical characteristics.
Unlike other electron-transfer materials, diphenoquinone compounds are known to have an exceptionally high compatibility with resins as well as a high electron mobility. On the other hand, diphenoquinone compounds tend to exhibit a strong color due to the long conjugated system within the molecule, and when used to form a photosensitive layer, diphenoquinone compounds absorb light that would otherwise reach the charge-generation material. As a result, the sensitivity of the photosensitive layer is decreased. Further, these compounds generate electrically stable radicals due to the symmetrical structure of their molecule skeletons. The radicals form electrical traps to hinder movement of electrons in a low electric field. Not only does this result in a reduced luminescence efficiency, and thus a reduced brightness of organic electroluminescence elements, but it also results in a high residual potential in the photosensitive layer of the photoreceptor.
One example of the electron-transfer material that has overcome the problem of reduced electron mobility in low electric fields is a compound described in Japanese Patent Laid-Open Publication No. Hei 9-34141, which has the structure shown by the following chemical formula 24:
The compound has a conjugated system involving three double bonds between an oxygen atom and a dicyanomethylene group within its molecular skeleton. Because of this relatively short conjugated system, the compound is capable of coloring only faintly and is thus less likely to absorb light.
Also, the dicyanomethylene group attached to one end of the quinone structure serves to keep the lives of the radicals produced during the movement of electrons short. As a result, the radicals are less likely to form traps even in a low electric field. However, movement of electrons is more restricted in this compound than in diphenoquinone compounds since its short conjugated system permits electrons to move within the molecule only over relatively short distances.
In practice, electrophotographic photoreceptors using the above compound are less than satisfactory when compared to the commercially available negative charge electrophotographic photoreceptors in terms of sensitivity and residual potential.
It is thus necessary for a practically useful electron-transfer material to meet two contradictory requirements: it must have a reduced ability to color and it must ensure a large degree of electron movement within the molecule. The former requirement is met by a short conjugated system in the chemical structure, whereas the latter is met by a long conjugated system provided by a larger molecular skeleton. A strong demand exists for a molecular structure that, aside from meeting these requirements, does not produce stable radicals.